Ice nanoclusters at hydrophobic metal surfaces

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1 e nnolusters t hydrophoi metl surfes ARTCLES ANGELOS MCHAELDES 1,2 * AND KARNA MORGENSTERN 3 1 Fritz-Her-nstitut der Mx-Plnk-Gesellshft, Frdyweg 4-6, D-1415 Berlin, Germny 2 London Centre for Nnotehnology nd Deprtment of Chemistry, University College London, London WC1E 6BT, UK 3 nstitut für Festkörperphysik, Leiniz University of Hnnover, Appelstr. 2, D Hnnover, Germny *e-mil: [email protected] Pulished online: 17 June 2007; doi:10.103/nmt140 Studies of the struture of supported wter lusters provide mens for otining rigorous moleulr-sle desription of the initil stges of heterogeneous ie nuletion: proess of importne to fields s diverse s tmospheri hemistry, strophysis nd iology. Here, we report the oservtion nd hrteriztion of metl-supported wter hexmers nd fmily of hydrted nnolusters heptmers, otmers nd nonmers through omintion of low-temperture snning tunnelling mirosopy experiments nd first-priniples eletroni-struture lultions. Aside from hieving unpreedented resolution of the yli wter hexmer the so-lled smllest piee of ie we identify nd explin hitherto unknown ompetition etween the ility of wter moleules to simultneously ond to sustrte nd to ept hydrogen onds. This ompetition lso rtionlizes previous struture preditions for wter lusters on other sustrtes. Few physil proesses re s uiquitous or feture more prominently in our dily lives thn the nuletion of wter into ie. Despite hving een studied sine ntiquity, the omplexity of the intermoleulr intertions etween wter moleules mens tht our moleulr-sle understnding of ie nuletion remins inomplete. This is prtiulrly true for heterogeneously tlysed nuletion in whih wter is prompted to nulete through the presene of n ie nuleting gent : mirosopi seed prtile of slt, snd or, in old-dpted orgnisms, so-lled ntifreeze proteins. As most of the ie rystls enountered in our dily lives re formed with the id of n ie nuleting gent, there is n impertive to etter understnd these proesses nd pressing need to understnd their relevne to nd impt on environmentl hemistry, strophysis nd iology, s well s other disiplines 1 4. The presene of the sustrte on whih wter lustering nd ie nuletion proeeds in heterogeneous nuletion rings with it hllenges nd opportunities. The key hllenge is the dded omplexity tht the sustrte introdues: intertions etween wter moleules in the nsent lusters my e ltered y the sustrte, leding to interesting effets suh s the formtion of new wter strutures not oserved in the gs phse, ltered H-ond strengths or different luster dynmis (see, for exmple, refs 5 ). The key opportunity the solid support rings, however, is the possiility of interrogting the properties of the dsored lusters with lrge rnge of surfe-sensitive proes,10. ndeed, surfe-sensitive spetrosopies (virtionl nd eletroni) nd diffrtion tehniques re regulrly pplied to provide detiled informtion on the properties of wter lusters nd wter overlyers on wide rnge of solid supports. And, when the sustrte is eletrilly onduting, suh s metl surfes re, there is the unique opportunity of diret rel-spe visuliztion of the lol struture of wter dlyers y mens of low-temperture snning tunnelling mirosopy 6 (LT-STM). ndeed LT-STM of wter dlyers on metls hs een extremely fruitful, reveling where individul wter moleules dsor 5 nd how they diffuse nd ggregte into wter lusters suh s dimers nd hexmers 5,11,12, s well s helping to identify the strutures of novel extended one-dimensionl 13,14 (1D) or qusi-2d 15 ie-like hins. However, LT-STM reports of wter on metls re sre nd muh remins to e lerned, prtiulrly with regrd to dsored hexmers, whih re of entrl importne euse they re the uilding loks of ommon ie, h. Here, we report omined LT-STM nd first-priniples density-funtionl theory (DFT) study in whih the initil stges of ie nuletion on the lose-pked (111) surfes of Cu nd Ag re explored. We report the oservtion nd hrteriztion of yli wter hexmers nd novel fmily of hydrted nnolusters heptmers, otmers nd nonmers. Aside from hieving unpreedented resolution of the yli hexmer the solled smllest piee of ie 16 we identify nd explin hitherto unknown ompetition etween the ility of wter moleules to simultneously ond to sustrte nd to ept H onds. This ompetition lso rtionlizes mny previous struture preditions for wter lusters on other sustrtes 7, Figure 1 shows typil imges otined fter dosing wter onto Cu(111) nd Ag(111) t low tempertures (17 K). The ehviour on eh surfe is similr, s is the ehviour of H 2 O nd D 2 O. Generlly, lrge morphous lusters nd numer of smll wter prtiles re oserved, s shown in Fig. 1. The morphous lusters ontin tens of wter moleules nd do not exhiit ny pprent order or reurrent similrities. The smller wter prtiles, however, lelled in Fig. 1 d ording to the numer of moleules thought to e ontined within them, pper in only four hrteristi representtions. At the lowest overges ( 0.05 ilyers (BL)), the wter prtiles re exlusively oserved (Fig. 1), wheres t higher overges ( 0.5 BL), they oexist with the lrger morphous lusters (Fig. 1,d). At ll overges exmined ( 1 BL), the wter prtiles remin s disrete nnostrutures nd do not gglomerte or olese to form extended strutures suh s the 1D or 2D strutures oserved on Cu(110) nd Pd(111) nture mterils VOL 6 AUGUST Nture Pulishing Group

2 ARTCLES 100 Å d Å 50 Å 50 Å Figure 1 Seleted STM imges of dsored wter lusters on Cu nd Ag., H 2 O on Cu(111) (120 mv nd 10 pa)., D 2 O on Ag(111) ( 55 mv nd pa)., H 2 O on Cu(111) (2 mv nd 11 pa). d, H 2 O on Cu(111) (74 mv nd 11 pa). n the rystlline prtiles oserved re irled nd in d they re lelled with the numer of wter moleules tht they omprise. 10 Å 10 Å 10 Å 10 Å Figure 2 High-resolution STM imges of dsored wter lusters., A H 2 O hexmer on Cu(111) (20 mv nd 11 pa). The inset shows the Cu(111) sustrte with tomi resolution ( 4 mv nd 2 na)., A D 2 O heptmer on Ag(111) (11 mv nd 2 na). The inset shows the Ag(111) sustrte with tomi resolution (15 mv nd 0.11 na)., A D 2 O otmer on Ag(111) ( 21 mv nd 2 na). d, A D 2 O nonmer on Ag(111) (11 mv nd 2 na). We note tht similr strutures re oserved with D 2 O nd H 2 O on oth Cu(111) nd Ag(111). or the muh-disussed ie ilyer strutures oserved on other metl surfes 6,,10,22 2. High-resolution STM imges suh s those shown in Fig. 2 nd DFT lultions led to the onlusion tht the smller prtiles oserved omprise 6 wter moleules, tht is, hexmers through to nonmers. From STM, hexgon of six protrusions is oserved for the smllest prtile (Fig. 2). The inset of the sustrte with tomi resolution indites tht the hexgon is pproximtely ligned with the lose-pked diretions of the sustrte nd the d dditionl protrusion t the ottom of the imge, n dsored wter monomer, leds to the suggestion tht the hexgonl pttern is wter hexmer. This ssignment is supported y DFT, whih finds tht on oth Cu(111) nd Ag(111) the lowest energy 6H 2 O luster is yli hexmer with eh H 2 O ting s single H-ond donor nd single H-ond eptor: so-lled homodromi wter luster. This DFT struture ws rrived t fter testing more thn 30 distint initil onfigurtions of the dsored H 2 O hexmer s well s severl simulted nneling initio moleulr dynmis simultions. n the low-energy struture identified, ll H 2 O moleules re loted lose to top sites: on Cu(111) the verge displement from the preise top sites is 0.11 Å nd on Ag(111), with the lrger lttie onstnt, it is 0.2 Å. nterestingly, the DFT struture of the hexmer exhiits notiele ukling in the heights of djent H 2 O moleules (Fig. 3, right). The vertil displement etween djent H 2 O moleules is 0.76 Å on Cu nd 0.67 Å on Ag. Furthermore, the six nerest-neighour O O distnes re not equl: they lternte etween two hrteristi vlues of 2.76 nd 2.63 Å on Cu nd 2.73 nd 2.65 Å on Ag. This symmetry-reking ond ltertion is reminisent of the lternting single nd doule C C onds in the Kekulé model of enzene (see the shemti digrm in Fig. 3). Thus, the wter hexmers identified here ould e desried s eing Kekulé-like. Let us turn now to the other smll wter prtiles oserved (Fig. 2 d). As the hexgonl rrngement of the moiety in Fig. 2 is still pprent ut now with n dditionl peripherl protrusion, we onlude tht this speies is wter heptmer. This ssignment is supported y STM mesurements in whih the seventh wter moleule n e moved to six different positions on the hexmer y eletron-indued mnipultion nd y DFT whih finds the struture shown in Fig. 3 to e the lowest-energy struture for 7H 2 O moleules on Cu(111) nd Ag(111). Aepting tht the two smllest rystlline prtiles re hexmers nd heptmers, it is resonle then to ssign the two remining speies (Fig. 2,d) to wter otmers nd nonmers, lthough we note tht n imge of n dsored nonmer on Cu(111) ws previously interpreted s trimer 11. The ssignments of the lrgest oserved lusters s otmers nd nonmers re gin supported y DFT (Fig. 3,d). ndeed, DFT revels tht for the otmer nd nonmer (s with 5 nture mterils VOL 6 AUGUST Nture Pulishing Group

3 ARTCLES H L H L L d 0.76 Tle 1 Seleted results of the test lultions of the energy differene, E (mev per H 2 O), etween the ukled nd plnr yli H 2 O hexmers on Cu(111). A positive E indites tht the ukled hexmer is more stle thn the plnr one, whih is lwys the se. Approh E (mev per H 2 O) Cu(111) PBE +122 Cu 10 Cluster PBE +170, +175 Cu 10 Cluster PBE0 +170, +173 Cu 10 Cluster B3LYP +201 Cu 10 Cluster MP2 +14, +16 Pseudopotentil plus plne-wve pproh. All-eletron with G(2df,pd ) sis set. All-eletron with G(3df,3pd ) sis set. All-eletron with G(2df,pd ) sis set. Figure 3 Optimized strutures nd seleted distnes (Å) otined from DFT for H 2 O lusters on Cu(111)., Top nd side view of the equilirium yli hexmer. d, Top views of the lusters with 7 H 2 O moleules. n some of the high/low H 2 O moleules re lelled with n H/L. The inset in shows shemti digrm of the Kekulé struture of enzene. The low-energy strutures otined from DFT on Ag(111) re similr. the heptmer), the dditionl H 2 O moleules dd to the low-lying H 2 O moleules of the entrl hexgon s H-ond eptors, tht is, there is preferene for the dditionl H 2 O moleules to ond to one of the two types of wter moleule in the hexmer. t is this preferene tht explins the hrteristi strutures oserved in the experiments for the otmer nd nonmer with wter moleules tthed only to next-nerest sites of the hexmer. The ukling of the dsored hexmer is importnt euse it rtionlizes the struture of the lrger lusters (otmers nd nonmers) tht form nd, s we show elow, sheds new light on the nture of interfil H onds. However, the ukling is not oserved y STM nd s STM simultions within the Tersoff Hmnn pproh 2 (not shown) indite tht the ukling in the equilirium hexmer struture should e pprent, we must question the vlidity of the theoretil predition. Therefore, we ompre on Cu(111) the energy of the equilirium ukled hexmer with n idel plnr hexmer in whih ll six H 2 O moleules re t the optimum height for H 2 O monomer dsorption. We find tht the hypothetil plnr hexmer is signifintly less stle thn the equilirium ukled hexmer: 122 mev per H 2 O. This reltive energy differene etween the two strutures is outside the typil solute errors in H-ond strengths ssoited with the Perdew, Burke nd Ernzerhof (PBE) funtionl used here ( 40 mev per H ond ording to refs 30 nd 31). Nonetheless, it is not inoneivle tht this differene is result of our hosen omputtionl set-up or exhnge-orreltion funtionl. Thus, we hve rried out series of tests with other DFT funtionls (PBE0 (ref. 32) nd the Beke three-prmeter Lee, Yng, Prr hyrid funtionl 33 (B3LYP)) nd with Møller Plesset perturtion theory to seond order (MP2): theoretil pprohes often onsidered to provide more relile energetis thn the DFT-PBE set-up used here 34. These tests, whih were rried out on Cu lusters, re reported in Tle 1 nd ll led to the sme onlusion: there is onsiderle energeti preferene (>100 mev per H 2 O) for ukling. There re severl possiilities s to why the ukled struture, whih is lerly fvoured y first-priniples theory nd rtionlizes the strutures oserved for the lrger wter lusters, is not pprent from the STM imges. One explntion, supported y previous STM experiments for H 2 O lusters on Ag(111) (ref. 35) nd DFT lultions for H 2 O on Pd(111) (ref. 36), is the influene of the eletri field from the STM tip, whih my reorient the moleules s they re imged. An lterntive, lthough not unrelted, explntion is tht wht is oserved in experiment (Fig. 2) my e dynmil verge of mny strutures smpled over the timesle of the STM mesurement (seonds) rther thn single equilirium groundstte struture. ndeed, very short ( 3 ps) initio moleulr dynmis simultion t 100 K for the hexmer on Cu(111) supports this suggestion, reveling tht its struture is highly flexile, prtiulrly with regrd to the heights of the wter moleules. So why does the ukling our nd wht does it tell us out interfil H onds? Let us return to the hypothetil plnr hexmer. n this hexmer, ll six wter moleules re equivlent nd thus so too is their intertion with the sustrte. Likewise, we would expet their intertions with eh other to e the sme. To exmine this we define quntity, ρ (see the ption of Fig. 4 for more detils), tht llows us to monitor how the eletron density tht lies ehind our eletroni-struture lultions rerrnges s the intertions etween dsored wter moleules re swithed on, tht is, ρ is speifi type of eletron density differene designed to revel the intertions etween wter moleules in the dsored lusters. A plot of ρ is shown in Fig. 4. As ntiipted, it shows tht the H 2 O H 2 O intertions etween the dsored moleules in the hypothetil plnr hexmer re indeed equivlent. Furthermore, the nture of the rerrngement is hrteristi of tht ssoited with H onding: with depletion (umultion) of density on the H (O) toms implited in the H onds, similr to the H ond in ie 37. We know, however, tht the plnr dsored hexmer on Cu (Ag) is 122 ( 76) mev per H 2 O less stle thn the equilirium dsored hexmer. This is prtly geometri effet s in this struture the wter moleules do not hve the optiml tetrhedrl onfigurtion for H onding with eh other. Through ukling the H 2 O moleules get loser to tetrhedrl rrngement nd, indeed, even in the sene of sustrte our lultions indite tht plnr yli hexgon gins 50 mev per H 2 O y ukling. Within the ukled dsorption struture the two types of wter moleules intert differently with the sustrte, s n e seen, for exmple, from the density of Kohn Shm eigensttes round eh type of wter moleule shown in Fig. 4. Here, it n e seen tht sttes loted round the low-lying dsored moleules re lower in energy thn sttes loted round the high-lying moleules nd, in prtiulr, sttes of 1 1 hrter on the low-lying moleules re shifted to lower energies through their intertion with the sustrte. Ogswr et l. hve rgued tht, for wter ilyer on Pt(111), this stiliztion is relted to polriztion of the nture mterils VOL 6 AUGUST Nture Pulishing Group

4 ARTCLES high H 2 O E E F (ev) low H 2 O d PDOS (e ev 1 ) Figure 5 Strutures of model wter ilyers on metl surfes nd some typil strutures of smll dsored wter lusters.,, Top views of the H-up () nd H-down () ilyer models for H 2 O on hexgonl metl surfes. A single H 2 O hexgon in eh type of ilyer is highlighted., Side view of the typil struture dopted for n dsored H 2 O dimer on metl surfe. d, Side view of typil struture dopted for n dsored H 2 O trimer. Figure 4 Eletroni strutures of wter hexmers on Cu(111).,, sosurfes of onstnt eletron density rerrngement ( ρ) for H 2 O hexmers on Cu(111) for hypothetil plnr hexmer () nd for the equilirium (ukled) hexmer (). ρ revels H 2 O H 2 O onding in the dsored lusters. t is defined s ρ = ρ 6H2 O/Cu + ρ Cu ρ 3H2 O /Cu ρ 3H2 O /Cu, where ρ 6H2 O/Cu nd ρ Cu re the eletron densities of the totl dsorption systems nd the isolted Cu surfes. ρ 3H2 O /Cu nd ρ 3H2 O /Cu re the eletron densities of two susets of the six dsored H 2 O moleules, s lelled in nd. Pink isosurfes orrespond to regions of eletron umultion nd green isosurfes to regions of eletron depletion in units of density hnge equl to e Å 3., Projeted density of sttes (PDOS) for the equilirium hexmer on Cu(111), projeted onto the high-lying (lk line) nd low-lying (red line) types of wter moleule. Peks tht re mostly of 1 1 hrter (s determined from inspetion of the density of the individul Kohn Shm eigensttes) re indited. The energy zero is the Fermi level (E F ). sustrte s eletron density whih minimizes the Puli repulsion etween the 1 1 oritl nd the sustrte 23. As the 1 1 oritl of H 2 O is lso implited when H 2 O ts s H-ond eptor, the low-lying H 2 O moleules re thus rendered poor H-ond eptors through their intertion with the sustrte. Essentilly we see tht there is ompetition etween the ility of H 2 O moleule to ond with the surfe nd its ility to t s H-ond eptor. t is this ompetition tht leds to the symmetry-reking ond ltertion in the hexmer struture with, s n e seen from the eletron density rerrngement plot in Fig. 4, the longer weker H onds formed when the low-lying H 2 O moleules t minly s H-ond eptors nd the stronger shorter H onds when the highlying H 2 O moleules t minly s H-ond eptors. We now disuss our results in the roder ontext of wter dsorption on solid surfes y first ompring the hexmers identified here with the hexmers tht uild dsored wter ilyers: the most ommonly disussed wter overlyers on metl surfes. n dsored ilyers, eh hexgon omprises two types of wter moleule: one tht lies pproximtely prllel to the surfe nd nother tht lies in the plne of the surfe norml. The ltter hs one OH ond tht does not prtiipte in the H-onded overlyer nd hs the option of direting this OH into the vuum ( H up model 22, Fig. 5) or t the surfe ( H down model 23, Fig. 5). DFT lultions hve een rried out for the H-up nd H-down ilyers on Cu(111) nd Ag(111) s well s for idelized dsored hexmers ut out of suh ilyers. We find tht the (non-relxed) hexmers ut out of the ilyers, with dsorption energies of 270 nd 20 mev per H 2 O on Cu nd Ag, respetively, re signifintly less stle thn the equilirium ukled Kekulé-like hexmers identified here, whih hve dsorption energies of 440 nd 416 mev per H 2 O on Cu nd Ag, respetively. Moreover, the inding energies of the equilirium hexmers identified here re essentilly identil to those in the extended 2D overlyers on eh surfe 3. This is noteworthy euse the wter moleules in the disrete hexmers hve fewer H onds per moleule (2/H 2 O in the isolted hexmers s opposed to 3/H 2 O in the ilyers), whih tells us tht simply ounting the numer of H onds in wter dlyers is not neessrily useful wy to judge their stilities. Next we disuss the dsorption of isolted hexmers on other metl surfes. Speifilly, we onsider how the lne etween H 2 O H 2 O nd H 2 O metl onding proeeds s we move to the surfes of more retive metls. To this end, DFT lultions hve een rried out for yli hexmers dsored on severl lose-pked metl surfes to the left of Ag in the periodi tle (Pd(111), Rh(111) nd Ru(0001)). t is known tht s we move from right to left ross the 4d trnsition series, the intertion etween wter nd the sustrte inreses,10,3. Thus, we would expet tht in the ompetition we hve identified here etween H 2 O metl onding nd the eptne of H onds t some stge it would no longer e fvourle for hexmers to srifie H 2 O metl onds so s to strengthen H onds. We find, preisely s ntiipted, tht the tendeny to ukle diminishes on going from Pd to Ru, ut it is only on Ru tht the plnr struture is more fvourle thn the ukled one. Speifilly, DFT lultions for initilly flt nd ukled hexmers on Pd, Rh nd Ru show tht fter relxtion the ukled struture is fvoured over the plnr one y 25 mev per H 2 O on Pd nd y 10 mev per H 2 O on Rh. On Ru, the plnr hexmer struture is fvoured over the ukled one y 10 mev per H 2 O, whih is onsistent with previous report of plnr hexmer on Ru (ref. 27). Thus, the ukling of the hexmers nd the ssoited Kekulé-like ltertion in the H 2 O H 2 O distnes seems to e rther generl phenomenon of wter hexmer dsorption. t is interesting to note tht this periodi vrition in the lne etween H 2 O H 2 O nd H 2 O metl onding identified here is reminisent of ion solvtion in wter where ompetition etween ion H 2 O nd H 2 O H 2 O onding exists, with ertin ions known s struture mkers nd 600 nture mterils VOL 6 AUGUST Nture Pulishing Group

5 ARTCLES others s struture rekers ording to the influene they hve on the first hydrtion sphere Finlly, we riefly onsider if the insight glened from this work n e used to rtionlize the strutures of dsored wter lusters on solid surfes in generl. We do not expet tht the ompetition identified here will e relevnt to ll sustrtes (in prtiulr, hydrophoi grphite surfes 43,44 nd ertin hydroxylted silite surfes in whih the nture of the intertion of wter with the sustrte is distintly different from tht experiened here), however, the ompetition does rtionlize previous struture preditions on wide rnge of sustrtes. For exmple, lultions of wter dimers on numerous metl (Pd (ref. 7), Pt (ref. 1) nd Ni (ref. 20), nd lso here we hve omputed dimers on Cu(111) nd Ag(111)) nd non-metl (NCl (refs 1,21) nd BF 2 (ref. 17)) surfes ll predit n symmetri ukled struture for the dimer with the H-ond eptor notiely further ( 0.5 Å) from the surfe thn the H-ond donor. The typil struture of n dsored H 2 O dimer is shown in Fig. 5, whih n now e understood y reognizing tht the intertion of wter moleule with sustrte diminishes its ility to ept H onds ut not neessrily its ility to donte H onds. Likewise, lultions for wter trimers on Ni(111) (ref. 20) nd trimers on Cu(111) omputed s prt of this study predit n symmetri struture in whih the H-ond eptor moleules intert wekly with the sustrte (Fig. 5d). Thus, the onlusion tht there is ompetition etween the ility of wter moleules to simultneously ond to sustrte nd to ept H onds hs some rod relevne eyond the wter lusters nd nole metls exmined here nd is likely to provide useful wy of thinking out the strutures of wter lusters on mny other solid sustrtes. METHODS The experiments reported here were rried out with ustom-uilt ultrhigh vuum STMs 45. The Cu(111) nd Ag(111) smples were lened y repetitive yles of Ne + sputtering nd nneling to 700 K. Wter of milli-q qulity (10 7 m), whih ws further purified under vuum y freeze thw yles, ws dosed onto the rystls through lek vlve while keeping the smple t 17 K. Mesurements on Cu(111) nd Ag(111) were rried out t 10 nd 5 K, respetively. The mjority of the lultions reported here involve DFT within the plne-wve superell pproh s implemented in the CASTEP ode 46 with ultrsoft pseudopotentils nd the PBE (ref. 47) exhnge-orreltion funtionl. As lrge superells, up to p(6 6), were used to ommodte the H 2 O lusters, thin three-lyer Cu nd Ag sls were used. Test lultions for wter lusters on sls of up to nine lyers thikness showed tht on the thiker sls dsorption energies nd strutures devited y <10 mev per H 2 O nd <0.1 Å, respetively. During struture optimiztions, the top lyer of metl toms ws free to relx. Monkhorst Pk k-point meshes with the equivlent of t lest 1 smpling within the surfe Brillouin zone of p(1 1) unit ell were used. 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Frish, M. J. et l. Gussin 03. Revision C.02 (Gussin, n., Wllingford, 2004). Aknowledgements We re grteful to M. Sheffler for vlule disussions. K.M. is grteful to the Deutshe Forshungsgemeinshft (DFG) for Heisenerg sholrship. This work ws onduted s prt of EURY sheme. See Correspondene nd requests for mterils should e ddressed to A.M. Competing finnil interests The uthors delre no ompeting finnil interests. Reprints nd permission informtion is ville online t nture mterils VOL 6 AUGUST Nture Pulishing Group